A lead storage battery has liquid electrolyte of dilute sulfuric acid employed therein, the concentration of which is generally determined by measuring specific gravity thereof. It is well known that the specific gravity of the electrolyte in the storage battery, when it is charged, is relatively greater and when discharged, relatively smallr. By way of a lead storage battery for a vehicle, for example it has specific gravity of 1.260 (20.degree.C) when completely charged and has that of 1.110 when charged by 25 percent. Thus, the charge and discharge of the lead storage battery has been generally examined by measuring the specific gravity of the electrolyte in the storage battery.
The prior art device for measuring the specific gravity of the electrolyte is generally of a floating type hydrometer which comprises a glass sleeve with a rubber hollow ball-like head fitted to the top of the sleeve, which is substantially similar to a squirt gun, and a float received in the glass sleeve and graduated on the surface thereof. One of the disadvantages of the prior art is to make its treatment troublesome because the operator must suck up the electrolyte by means of the glass sleeve with the rubber head through the opening of the battery case. Another disadvantage of the prior art is to impair the clothes on the body of the operator and the matters around the storage battery, such as the instruments and the likes, because the electrolyte is corrosive. In view of the foregoing disadvantages, there has been proposed an arrangement wherein a floating ball of constant specific gravity is floated on the electrolyte in the battery case for allowance of vertical movement therein so that the operator can examine the level position of the float ball through the graduated wall of the battery case. However, in some storage batteries, such as those for vehicles their cases have been formed of opaque material, such as ebonite, polypropylene and the likes and as a result the above-described arrangement cannot be applied for such storage batteries.
Since the electrolyte in the battery case is consumed due to decomposition of water in the electrolyte which occurs during charge of the battery and evaporation of water and is reduced in level, the operator is often required to supplement water in the electrolyte, because the storage battery is otherwise reduced in its life and performance. However, the storage battery with opaque case as above-mentioned requires the operator to check or monitor the level of the electrolyte in the case through the opening thereof to cause the operation to be troublesome and the level of the electrolyte in the case to fail to be checked.